If anyone has done this with the modern cards I'd like some input. I would like to add a second display and video card to my system, not to spread the current desktop over so much as to have a secondary workspace displayed simultaeneously. The second display need need not have the horsepower of my current card (HD4870) but would like something with enough horsepower that it could be used as a folding or crypto engine when not being used for display purposes but not break the bank. I'm told that the new generation of cards don't like brand mix-n-match too well so I guess that would leave another ATI solution. Any suggestions?

Most of the more recent video cards are perfectly adequate to drive two displays on their own. I'm currently using one 8800 GTS to drive a 24" and a 22" monitor through the dual outputs on the card and it's just fine that way.
But before that I was using an 8600 GTS, with great results. And I'm not just opening web pages and text files on those monitors either. I do 3D graphics and I often have 3ds max or Lightwave opn on one monitor and Photoshop on another - fairly OGL-intensive things, and the setup never chokes. I've also run 3ds max on one and Lightwave on the other, maximized, loaded with polygons and textures. :)
But if you do want to do it with separate video cards, I think they have to be identical makes and models.
I'm not that familiar with the HD4870, except I know it's very powerful. Assuming it has dual DVI outputs, that's well more than enough for what you want, believe me.